Soil Loading Rate by Texture Reference

Why this matters

The size of a drainfield is set by two inputs: the design flow in gallons per day and the soil loading rate in gallons per square foot per day. Soil loading rate is read off a table keyed to soil texture, which is the relative percentages of sand, silt, and clay in the soil horizon where the trench bottom will sit. If you read the wrong row of the table, the field is undersized and fails years early, or oversized and the customer pays for trenches that will never be loaded. The texture call is the highest-leverage decision in the entire design. Get it right and the system lasts; get it wrong and no amount of pump, pipe, or aerator fixes it later.

How soil texture is determined

The USDA-NRCS soil texture triangle defines 12 soil classes by sand-silt-clay percentages. A site evaluator (or designer, depending on state) digs a soil profile (typically a backhoe pit) to the depth of the planned trench bottom plus the required vertical separation to the limiting layer or seasonal high water table. Texture is called at each horizon by the ribbon test (rolling moist soil between thumb and finger) and, where required, confirmed by sieve and hydrometer analysis in a lab.

The horizon at the trench-bottom elevation is the one that drives loading rate. A site with sandy loam at the surface but heavy clay at 30 inches must use the clay loading rate, not the sandy loam, because the trench bottom will sit in the clay.

Typical loading rates by texture class

State codes set the values; they cluster within a narrow band across jurisdictions because the underlying soil science is the same. Loading rates below are the band you see across most U.S. state codes; always pull the actual table from the AHJ before designing.

• Sand, coarse sand: high loading rate, but pressure distribution is required by many states above a code-defined sand threshold because effluent moves too fast for gravity gravity systems to achieve treatment before it reaches groundwater.
• Loamy sand, sandy loam: high to moderate loading rate, gravity acceptable in most states.
• Loam, silt loam: moderate loading rate, this is the textbook gravity-field soil.
• Sandy clay loam, clay loam, silty clay loam: low loading rate, gravity acceptable but trenches sized larger; consider pressure distribution to extend service life on borderline soils.
• Sandy clay, silty clay, clay: very low loading rate, gravity may not be allowed; mound or other elevated systems often required.

The exact values, in gallons per square foot per day, are published in each state's on-site sewage rules. Examples of published rates:

• North Carolina 15A NCAC 18A .1955 publishes loading rates by soil group SG-I through SG-IV.
• Virginia 12 VAC 5-610-594 publishes loading rates by soil texture class.
• Washington WAC 246-272A-0230 publishes loading rates by soil treatment Group 1 through 6.
• Oregon OAR 340-71-0220 publishes loading rates by soil category.

Required vertical separation

Loading rate is only one input. The state code also defines the required vertical separation between trench bottom and the limiting layer (a fragipan, bedrock, or seasonal high water table). Typical separation requirements:

• 18 to 24 inches to seasonal high water table for conventional gravity systems
• 12 inches to bedrock or fragipan
• Greater separation for sandy soils where the treatment depth is the only barrier between effluent and groundwater

A site that meets loading rate but fails vertical separation must be elevated (mound, at-grade, or fill system) or the design is rejected.

Percolation test versus soil evaluation

Older codes use the percolation test (the falling-water test in a soil pit, measured in minutes per inch) as the primary input. Newer codes use soil evaluation (texture, structure, color mottling for water table evidence) and treat the perc test as confirmatory or eliminate it entirely. The trend is toward soil evaluation because perc tests have high site-to-site variance and are easily gamed by pre-soaking technique. Know which method your AHJ requires; some states still mandate both.

Field calls common mistakes

• Calling sandy loam where the actual horizon is loamy sand. The first holds water more; the second moves it faster. Texture call by ribbon test takes practice and a state-certified evaluator on staff is worth the licensing cost.
• Calling the surface horizon instead of the trench-bottom horizon. The soil at 6 inches is rarely the soil at 36 inches.
• Ignoring color mottling. Gray and orange mottling in the profile indicates seasonal water table, even if the day of the test is dry. The state code requires the mottling depth to be used as the high water table indicator.
• Skipping the second pit. Most state codes require multiple soil profile pits across the field area. A single pit can miss a transition that runs across the proposed trench layout.

Documentation

For every site, the soil evaluation record should include:

• Pit location plotted on the site map
• Horizon-by-horizon description (depth, texture, structure, consistence, color, mottling)
• Limiting layer depth and type
• Seasonal high water table depth
• Loading rate selected and the code citation
• Vertical separation calculated against the proposed trench-bottom elevation
• Designer signature and license number where the state requires it

References

• USDA NRCS Soil Survey Manual, Handbook 18 (national reference for soil texture classification)
• USEPA Onsite Wastewater Treatment Systems Manual, EPA/625/R-00/008
• North Carolina 15A NCAC 18A .1955, Site and Soil Evaluation
• Virginia Sewage Handling and Disposal Regulations 12 VAC 5-610, Article 6
• Washington WAC 246-272A-0230, Soil and Site Evaluation
• Oregon OAR 340-71-0220, Site Evaluation
• USDA NRCS National Engineering Handbook Part 631, Geology, Chapter 14 (Soil and Site Investigation)